1. Field of the Invention
The present invention relates to a method for manufacturing an artificial ultra-lightweight aggregate using raw sewage sludge, and more particularly, to a method for manufacturing an artificial ultra-lightweight aggregate having a specific gravity of 0.8-0.9 using raw sewage sludge, dry steel sludge and clay.
2. Background of the Related Art
Generally, lightweight aggregates mean aggregates having a unit weight of about 0.8-1.2 tonf/m3 as indicated in KSF 2505 (test method for unit weight of aggregate). These lightweight aggregates are broadly divided into natural aggregates and artificial aggregates. In Korea, however, a large amount of natural aggregates in a usable form is not available, and the mass production of artificial lightweight aggregates does not yet meet the demand.
Generally, artificial aggregates are manufactured using raw materials, such as expandable shale, expandable clay, fly ash, and the like, and should satisfy various requirements, including clear appearance, high strength and excellent durability. In addition, the artificial lightweight aggregate should have suitable particle size and unit weight. At the same time, the artificial lightweight aggregate should not contain harmful substances having an adverse effect on concrete and iron materials, and any change in the quality of the artificial lightweight aggregate attributable to manufacturing processes should be small.
Generally, methods for molding lightweight aggregates include injection molding, tape seating and plastic extrusion. Among these molding methods, plastic extrusion is the most economic method for mass production of lightweight aggregates, and particularly, the water content of a clay body in the plastic extrusion is highly important. If the water content exceeds 25%, molded bodies of lightweight aggregates will stick to each other, and if it is less than 20%, the plasticity of the clay body will excessively increase, so that an extruder is overloaded and extrusion becomes impossible.
Recently, studies have been conducted to manufacture eco-friendly aggregates at low cost using industrial wastes, such as fly ash, paper ash, coal fly ash, paper making sludge, waste incinerator fly ash, and wastewater sludge, which are increasingly generated in various industrial fields.
As a result of such studies, methods have been suggested for manufacturing artificial lightweight aggregates by mixing paper making sludge, paper making sludge incinerator fly ash, mining waste rock, sewage sludge or coal fly ash mainly with clay and then sintering the mixture. Examples of these methods include: Korean Patent Publication No. 96-11333, entitled “Artificial Lightweight Aggregate and Manufacturing Method Thereof”; Korean Patent Registration No. 10-208779, entitled “Lightweight Aggregate Having Coating Film Formed on Outer Surface and Manufacturing Method Thereof;” Korean Patent Registration No. 10-240943, entitled “Method for Manufacturing Porous Lightweight Building Material Using Red Mud;” Korean Patent Publication No. 1999-88360, entitled “Method for Manufacturing Artificial Lightweight Aggregate Having Low Specific Gravity Through Control of Dry Specific Gravity.” Also, as a prior patent application using sewage sludge, there is Korean Patent Publication No. 2002-0084975, entitled “Method for Manufacturing Calcined Building Material Using Sewage Sludge.”
In particular, the prior methods for manufacturing lightweight aggregate using sewage sludge utilize dry sewage sludge, but not raw sewage sludge, and involve excessive energy costs for drying sewage sludge and additional equipment for conducting the drying process, which increases equipment cost, and thereby greatly increases the unit cost of the aggregate product. Thus, these prior methods are not cost-effective and, in fact, hardly put into practical use. Meanwhile, the prior method for manufacturing the calcined building material using sewage sludge employs sewage sludge after drying, but has a shortcoming in that it does not consider water content important for the production of aggregates, because it uses a molding process other than extrusion molding suitable for the production of lightweight aggregates. In addition, the prior technology relating to ultra-lightweight aggregates having a specific gravity of less than 1 is insufficient.
Furthermore, as one of factors of defects caused during cast-in-place concrete construction, water is frequently added to concrete in situ, because the slump loss of concrete occurs during transport. This can result in strength reduction, material separation, drying shrinkage and the like. For this reason, there is a need for the production of aggregate capable of providing concrete that has low slump loss, and thus maintains suitable fluidity for a long period of time.